It is being stated with greater frequency that this country is an island as regards the need for importation of various raw materials essential to the health and growth of the nation's industry. Typical of such raw materials are various elements such as chromium, manganese, nickel, vanadium, molybdenum, tungsten and columbium used in the steel industry, the major portion of which are derived from foreign sources. The constant political turmoil in many countries where there are significant or major sources of such elements makes the availability of such elements unpredictable. In addition, many of these elements are scarce and frequently difficult to obtain in desired quantities. It's generally forecast that the situation may continue for some time.
There is available in this country one element which is available in almost limitless supply and which can be effectively used to replace, at least in part, such critical elements as chromium, manganese, nickel, molybdenum, vanadium, tungsten, columbium and the like. That element is boron, and it must be used more extensively to develop and produce boron steels which would replace alloy steels in certain critical applications.
Boron steels are not new. The original concept of using small amounts of boron to increase the hardenability of steel was conceived in the mid-twenties, with the first commercial applications coming in the mid-thirties.
Conventionally, boron steels have been made by ingot casting aluminum-killed steels containing very small amounts of boron, e.g. at least about 0.0030% B for low carbon (about 0.1% C) steels and at least about 0.0005% for high carbon (0.6% C) steels. The boron hardenability effect is only achieved when boron is in the so-called soluble or free uncombined state, i.e. not combined with oxygen, nitrogen or carbon in the steel. However, boron has a great affinity for oxygen and nitrogen, and these gases in the steel must either be removed or controlled if the cast steel is to contain the necessary amount of soluble boron to provide its full hardenability effect.
In order to enable the boron to be present in the steel in an uncombined state, elements such as aluminum, titanium and zirconium, which have a greater affinity for oxygen and nitrogen than boron, have been included in boron alloying additives to protect the boron from such gases. Aluminum, in addition to providing proper deoxidation of steel and protecting boron from oxygen in steel, provides a fine grained steel. However, there are a number of problems associated with aluminum-killed steels.
Alumina inclusions remaining in steel deoxidized with aluminum are detrimental to the physical properties of steel. In addition, high alumina residuals in steel as a result of aluminum deoxidation practice provide undesirable surface characteristics in that the surface is very rough in both ingot and continuously cast steel and needs conditioning for removing the roughness. When such steel is rolled, surface defects are present in the rolled product by reason of these surface and subsurface inclusions.
In continuous casting of aluminum-killed steels, the alumina formed during deoxidation deposits in the nozzle of the tundish, clogging the nozzle. The flow of molten steel is thereby restricted with evantual blockage. The nozzle clogging problem is associated with the total aluminum content of the steel, and the level of aluminum at which nozzle blockage occurs depends upon the size of the nozzle, the smaller the diameter of the nozzle, the lower the aluminum content which will cause blockage. For example, aluminum levels greater than about 0.007% can cause blockage of one inch diameter nozzles. Thus, it is difficult to cast aluminum-killed steels in a continuous caster and more particularly in a multiple strand billet caster having metering nozzles of less than one inch diameter.